Resist material and nanofabrication method

ABSTRACT

A resist material and a nanofabrication method provide high-resolution nanofabrication without an expensive irradiation apparatus using, for example, electron beams or ion beams. That is, the resist material and the nanofabrication method provide finer processing using exposure apparatuses currently in use. A resist layer of an incompletely oxidized transition metal such as W and Mo is selectively exposed and developed to be patterned in a predetermined form. The incompletely oxidized transition metal herein is a compound having an oxygen content slightly deviated to a lower content from the stoichiometric oxygen content corresponding to a possible valence of the transition metal. In other words, the compound has an oxygen content lower than the stoichiometric oxygen content corresponding to a possible valence of the transition metal.

This application claims priority to Japanese Patent Application NumberJP2002-046029, filed Feb. 22, 2002, and Japanese Patent ApplicationNumber JP2002-297893, filed Oct. 10, 2002, which are incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to inorganic resist materials andnanofabrication methods with the resist materials and, particularly, toa resist material and a nanofabrication method allowing high-precisionnanofabrication with an exposure source over ultraviolet to visiblelight region.

BACKGROUND ART

The latest lithography for nanofabrication in, for example,semiconductors, optical devices, and magnetic devices requirespatterning precision on the order of tens of nanometers or less. Toachieve such high-precision patterning, intensive development has beenmade in various fields such as light sources, resist materials, andsteppers.

Effective approaches to enhance the dimensional precision ofnanofabrication include use of shorter wavelengths and convergentelectron or ion beams in an exposure source. However, short-wavelengthexposure sources and convergent electron or ion beam irradiation sourcesare so expensive that these sources are unsuitable for providing lessexpensive devices.

To enhance the dimensional precision of machining with the same exposuresource as that in exposure apparatuses currently in use, otherapproaches have been proposed such as improvements in illuminationmethods and use of a special mask referred to as a phase shift mask.Further approaches have been attempted which include methods with amultilayer resist or an inorganic resist.

An exposure method has generally been employed which involves organicresists such as novolac resists and chemically amplified resists withultraviolet light as an exposure source. Organic resists, which areversatile, have extensively been used in the field of lithography.However, their large molecular weight results in an unclear pattern atthe boundary between exposed and unexposed areas. This isdisadvantageous from the viewpoint of enhancing the precision ofnanofabrication.

In contrast, inorganic resists, which have a low molecular weight,provide a clear pattern at the boundary between exposed and unexposedareas, and have the possibility of achieving high-precisionnanofabrication compared to organic resists. For example, Jpn. J. Appl.Phys., Vol. 30 (1991), p. 3246 introduces a nanofabrication method with,for example, MoO₃ or WO₃ as a resist material and ion beams as anexposure source; and Jpn. J. Appl. Phys., Vol. 35 (1996), p. 6673introduces a method with SiO₂ as a resist material and electron beams asan exposure source. Furthermore, SPIE, Vol. 3424 (1998), p. 20introduces a method with chalcogenide glass as a resist material and 476and 532 nm lasers and ultraviolet light from a mercury-xenon lamp asexposure sources.

The use of electron beams as an exposure source can be combined withmany kinds of inorganic resist materials, as described above, while onlychalcogenide has been reported as a material corresponding toultraviolet or visible light. The reason is that inorganic resistmaterials proposed other than chalcogenide that are transparent toultraviolet or visible light have significantly low absorbance, which isunsuitable for practical use.

Chalcogenide has the advantage of allowing ultraviolet or visible lightand therefore exposure apparatuses currently in use, but has the problemof containing some agents harmful to humans, such as Ag₂S₃, Ag—As₂S₃,and Ag₂Se—GeSe.

On the other hand, photolithography with ultraviolet or visible lightare extensively applied to the manufacture of various devices such assemiconductor devices, such as dynamic random access memory (DRAM),flash memory, central processing units (CPUs), and application specificICs (ASICs); magnetic devices, such as magnetic heads; displays, such asliquid crystal displays, electroluminescent (EL) displays, and plasmadisplay panels (PDPs); optical devices, such as optical recording mediaand optical modulation elements. Examples of these devices are compactdiscs (CD, which is a registered trademark), which are read-only opticaldiscs as typified by DVDs. The structure of an optical disc will bedescribed below.

An optical disc essentially includes an optically transparent substrateof, for example, polycarbonate, which has a main surface with a fineirregular pattern of, for example, pits and grooves representinginformation signals. The main surface is covered with a thin reflectivefilm of a metal such as aluminum, which is further covered with aprotective film.

Such a fine irregular pattern on the optical disc is formed with astamper having a high-precision fine irregular pattern through a processof transferring the pattern onto the substrate faithfully and readily. Amethod for preparing the stamper will be described below.

For example, a glass substrate with a sufficiently smooth surface isdisposed on a rotating platform. A photoresist, which is photosensitive,is applied onto the glass substrate rotating at a predeterminedrotational speed. The rotation spreads the photoresist over the glasssubstrate, so that the glass substrate is entirely spin-coated. Thephotoresist is exposed to recording laser light in a predeterminedpattern to form a latent image corresponding to information signals. Thephotoresist is then developed with a developer to remove an exposed orunexposed area, thereby providing a resist master with the predeterminedirregular pattern of the photoresist. Metal is further deposited on theirregular pattern of the resist master by a process such aselectroplating to transfer the irregular pattern to the metal. Themetal, which is a stamper, is separated from the resist master.

The stamper is used to duplicate a large number of substrates made ofthermoplastic resin, such as polycarbonate, by known transferringprocesses such as injection molding. Each of the substrates is thencovered with, for example, a reflective film and a protective film tocomplete an optical disc.

The capacity of information recordable on the optical disc depends onthe density of pits or grooves that can be formed. In other words, thecapacity of information recordable on the optical disc depends on thefineness of the irregular pattern formed by cutting, namely, exposing aresist layer to laser light to form a latent image.

For example, a stamper for read-only DVDs (DVD-ROMs) has a spiral pitstring with a minimum pit length of 0.4 μm and a track pitch of 0.74 μm.An optical disc 12 cm in diameter, produced with a stamper as a mold,has an information capacity of 4.7 GB per side.

Production of optical discs with such a structure requires a resistmaster prepared by a lithography process using a laser with a wavelengthof 413 nm and an objective lens with a numerical aperture (NA) ofapproximately 0.90 (for example, 0.95).

With the current rapid progress in information and communicationtechnology and image-processing technology, optical discs as describedabove are facing the task of achieving a recording capacity severaltimes higher than the current capacity. For example, next-generationoptical discs with a diameter of 12 cm, an extension of digitalvideodiscs, are required to attain an information capacity of 25 GB perside by conventional signal processing. To meet this requirement, theminimum pit length and track pitch of the optical discs must be reducedto approximately 0.17 μm and 0.32 μm, respectively.

The minimum pit length P (μm) in exposure is represented by equation (1)below:P=K·λ/NA  (1)where λ (μm) represents the wavelength of the light source; NArepresents the numerical aperture of the objective lens; and Krepresents a proportional constant.

The wavelength λ of a light source and the numerical aperture NA of anobjective lens are parameters depending on the specification of a laserapparatus. The proportional constant K is a parameter depending on acombination of the laser apparatus and the resist layer.

In the production of the above optical discs, such as DVDs, setting thewavelength to 0.413 μm and the numerical aperture NA to 0.90 leads to aminimum pit length of 0.40 μm, then providing a proportional constant Kof 0.87 from equation (1) above.

In general, a shorter laser wavelength is effective to achieve the finepit described above. That is, in the case of the same proportionalconstant K and, for example, NA=0.95, a light source with a laserwavelength λ of 0.18 μm is required to provide the minimum pit length ofapproximately 0.17 μm, which is necessary for high-density optical discswith a recording capacity of 25 GB per side.

The wavelength of 0.18 μm required in this case is shorter than awavelength of 193 nm of an ArF laser, which is being developed as alight source for next-generation semiconductor lithography. An exposureapparatus achieving such a short wavelength requires special opticalcomponents, such as the lens, as well as a special laser as a lightsource, thus becoming extremely expensive. In increasing the opticalresolution to address nanofabrication, an approach based on a shorterexposure wavelength λ and a larger numerical aperture NA is quiteunsuitable for production of inexpensive devices due to the followingreason: this approach inevitably requires the replacement of theexposure apparatuses currently in use with expensive exposureapparatuses because the exposure apparatus in use cannot keep up withthe advances in nanofabrication.

The present invention is proposed to solve such conventional problems.An object of the present invention is to provide a resist materialallowing high-precision nanofabrication without an expensive irradiatingapparatus using, for example, electron beams or ion beams. Anotherobject of the present invention is to provide a nanofabrication methodallowing finer processing with exposure apparatuses currently in use andthe resist material.

DISCLOSURE OF INVENTION

As described above, completely oxidized transition metals such as MoO₃and WO₃ are conventionally used as a resist material. These metals,however, present difficulty in nanofabrication by exposure withultraviolet or visible light because these metals are transparent toultraviolet or visible light to exhibit significantly low absorption.

As a result of study on this problem, the present inventors have foundthe possibility of applying transition metal oxides to a resist materialand a nanofabrication method. When the oxygen content of a transitionmetal oxide even slightly deviates from the stoichiometric oxygencontent, the oxide absorbs a large amount of ultraviolet or visiblelight, which changes the chemical characteristics of the oxide. Thischange increases the proportional constant K in equation (1) above toreduce the minimum pit length P.

The resist material according to the present invention has been inventedbased on the above knowledge. The resist material includes anincompletely oxidized transition metal having an oxygen content lowerthan the stoichiometric oxygen content corresponding to a possiblevalence of the transition metal.

The nanofabrication method according to the present invention includesthe steps of depositing a resist layer of a resist material including anincompletely oxidized transition metal having an oxygen content lowerthan the stoichiometric oxygen content corresponding to a possiblevalence of the transition metal on a substrate; exposing the resistlayer selectively; and developing the resist layer to be patterned intoa predetermined form.

The incompletely oxidized transition metal herein is defined as acompound having an oxygen content deviated to a lower content than thestoichiometric oxygen content corresponding to a possible valence of thetransition metal. In other words, the compound has an oxygen contentlower than the stoichiometric oxygen content corresponding to a possiblevalence of the transition metal.

An incomplete oxide containing plural kinds of transition metalsprobably have a crystal structure in which one transition metal atomsare partially replaced with the other transition metal atoms. Such anoxide is determined to be incomplete according to whether the transitionmetals have oxygen contents lower than their possible stoichiometricoxygen contents.

The incompletely oxidized transition metal used as the resist materialof the present invention absorbs ultraviolet or visible light to allowexposure without a special exposure source using, for example, electronbeams or ion beams. Furthermore, the incompletely oxidized transitionmetal is a low molecular metal to provide a clearer boundary betweenunexposed and exposed areas than a polymeric organic resist. Thistransition metal can be used as a resist material to provide ahigh-precision resist pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exposure apparatus used in ananofabrication method according to the present invention.

FIG. 2 is a characteristic diagram indicating the relationship betweenthe irradiation power of a light source for exposure and the differencein etching rate between exposed and unexposed areas in exposing a resistlayer of a resist material of the present invention.

FIGS. 3A to 3C are characteristic diagrams showing examples ofirradiation patterns in the exposure process. FIGS. 3A and 3B showexamples of irradiation pulses; FIG. 3C shows an example of continuouslight.

FIGS. 4A to 4D are schematic sectional views illustrating the relevantpart of a two-layer resist process. FIG. 4A illustrates the step ofdepositing first and second resist layers; FIG. 4B illustrates the stepof patterning the first resist layer; FIG. 4C illustrates the step ofetching the second resist layer; and FIG. 4D illustrates the step ofremoving the first resist layer.

FIG. 5 is a photograph observed by SEM and showing a developed resistlayer of incompletely oxidized tungsten.

FIG. 6 is a photograph observed by SEM and showing a developed resistlayer of incompletely oxidized molybdenum.

FIG. 7 is a process chart of producing an optical disc by ananofabrication method according to the present invention.

FIG. 8 is a photograph observed by SEM and showing a developed resistlayer of incompletely oxidized tungsten and molybdenum.

FIG. 9 is a photograph showing a pit pattern on a surface of an opticaldisc with a recording capacity of 25 GB produced in Example 3.

FIGS. 10A to 10C are photographs showing the signal evaluation of theoptical disc with a recording capacity of 25 GB produced in Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

A resist material and a nanofabrication method according to the presentinvention will now be described in detail with reference to thedrawings.

The resist material according to the present invention is anincompletely oxidized transition metal. Herein the incompletely oxidizedtransition metal is defined as a compound having an oxygen contentdeviated to a lower content from the stoichiometric oxygen contentcorresponding to a possible valence of the transition metal. In otherwords, the incompletely oxidized transition metal is defined as acompound having an oxygen content lower than the stoichiometric oxygencontent corresponding to a possible valence of the transition metal.

The oxidized transition metal will now be exemplified by chemicalformula MoO₃. When the oxidation state of the chemical formula MoO₃ isrepresented by composition Mo_(1-x)O_(x), x=0.75 indicates a completeoxide whereas 0<x<0.75 indicates an incomplete oxide having an oxygencontent lower than the stoichiometric oxygen content.

Some transition metals can form oxides with different valences. For suchmetals, the present invention is limited to incompletely oxidizedtransition metals having an actual oxygen content lower than thestoichiometric oxygen content corresponding to the possible valences ofthe transition metals. For example, molybdenum oxide is most stable inthe sexavalent state (MoO₃) described above, and can also be present inthe divalent state (MoO). When MoO is represented by compositionMO_(1-x)O_(x), 0<x<0.5 indicates an incomplete oxide having an oxygencontent lower than the stoichiometric oxygen content. The valences ofthe transition metal oxides can be analyzed with commercially availableanalytical instruments.

These incompletely oxidized transition metals absorb ultraviolet orvisible light to change their chemical characteristics by theirradiation of ultraviolet or visible light. This change causesselectivity in the resist, that is, a difference in etching rate betweenexposed and unexposed areas during the developing step, although theresist is an inorganic resist (this will be described below in detail).In addition, particles in a resist material film of an incompletelyoxidized transition metal are so small as to provide a clear pattern atthe boundary between exposed and unexposed areas, resulting in highresolution.

The incompletely oxidized transition metals change their characteristicsas a resist material according to the degree of oxidation. Thus, theoptimum degree of oxidation may be selected in each case. For example,an incompletely oxidized transition metal having an oxygen content muchlower than the stoichiometric oxygen content of the transition metaloxidized completely has disadvantages such as higher irradiation powerfor the exposure step and a longer developing time. Therefore, theincompletely oxidized transition metal preferably has an oxygen contentslightly lower than the stoichiometric oxygen content of the transitionmetal oxidized completely.

Examples of transition metals for the resist material include Ti, V, Cr,Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru, and Ag. Among these, Mo, W,Cr, Fe, and Nb are preferable. Furthermore, Mo and W are more preferablein that significant changes occur by ultraviolet or visible light.

In the present invention, the incomplete oxide may contain one, two,three or more transition metals, or another element except transitionmetals. In particular, the incomplete oxide preferably contains pluralkinds of metal elements.

When the incomplete oxide contains not only one but also two or three ormore transition metals, the incomplete oxide probably has a crystalstructure in which one transition metal atoms are partially replacedwith the other transition metal atoms. Such an oxide is determined to beincomplete according to whether the transition metals have oxygencontents lower than their possible stoichiometric oxygen contents.

Available elements except transition metals include Al, C, B, Si, andGe, and at least one of these elements can be used. A combination of twoor more kinds of transition metals or addition of an element other thantransition metals provides an incompletely oxidized transition metalcontaining smaller crystal particles. This forms a clearer boundarybetween exposed and unexposed areas, leading to significant increases inresolution and exposure sensitivity.

The above resist materials may be prepared by sputtering in an Ar+O₂atmosphere with a target containing a predetermined transition metal.For example, the flow rate of O₂ into a chamber is set to 5% to 20% ofthe total gas flow rate, and the gas pressure is set to 1 to 10 Pa,which are employed in normal sputtering.

Next, a nanofabrication method using the above resist materials will bedescribed.

The nanofabrication method of the present invention, for example,includes the steps of depositing a resist material of an incompletelyoxidized transition metal on a substrate to form a resist layer;exposing the resist layer selectively; and developing the resist layerto form a predetermined pattern. An example will now be described inwhich the nanofabrication method of the present invention is applied toa cutting step of a resist master for optical discs. Of course, thenanofabrication method of the present invention is not limited to thefollowing example and is also applicable to nanofabrication of variouselectronic devices such as semiconductor devices, optical devices,displays, and magnetic devices.

[Step of Forming Resist Layer]

A resist layer of an incompletely oxidized transition metal is depositedon a sufficiently smooth surface of a substrate. Depositing methodsinclude, for example, sputtering in an argon-oxygen atmosphere using asputtering target of an elemental transition metal. This method cancontrol the degree of oxidation of an incompletely oxidized transitionmetal by changing the concentration of oxygen gas in a vacuumatmosphere. An incompletely oxidized transition metal containing two ormore kinds of transition metals may be deposited by sputtering while thesubstrate is constantly rotated over different kinds of sputteringtargets to mix the different transition metals. The individual chargingpowers of the sputtering targets can be changed to control the mixtureratio.

In addition to the above sputtering in an oxygen atmosphere using ametal target, a resist layer of an incompletely oxidized transitionmetal may be deposited by sputtering in a normal argon atmosphere usinga target of an incompletely oxidized transition metal containing adesirable amount of oxygen.

Furthermore, a resist layer of an incompletely oxidized transition metalcan readily be deposited by evaporation, as well as sputtering.

The substrate may be composed of, for example, glass, plastic such aspolycarbonate, silicon, alumina-titanium carbide, or nickel.

The resist layer may have any thickness. For example, the resist layermay have a thickness within the range of 10 to 80 nm.

[Step of Exposing Resist Layer]

The substrate covered with a resist layer (hereinafter referred to as aresist substrate 1) is disposed on a turntable 11 of an exposureapparatus showed in FIG. 1 such that the surface having the resist layerfaces upward.

This exposure apparatus has a beam source 12, which emits, for example,laser light capable of exposing the resist layer. The laser light isfocused through a collimator lens 13, a beam splitter 14, and anobjective lens 15 to be incident on the resist layer of the resistsubstrate 1. The light reflected by the resist substrate 1 is thenfocused through the beam splitter 14 and a focusing lens 16 onto aseparate photodetector 17. The separate photodetector 17 detects thelight reflected by the resist substrate 1 to generate focus errorsignals 18 from the detection results and send the focus error signal 18to a focus actuator 19. The focus actuator 19 controls the verticalposition of the objective lens 15. The turntable 11 has a feed mechanism(not shown in the drawing), which can precisely change the exposureposition of the resist substrate 1. A laser drive circuit 23 controlsthe beam source 12 according to data signals 20, reflected lightintensity signals 21, and tracking error signals 22 to perform exposureor focusing. A spindle motor control system 24 is provided on thecentral axis of the turntable 11. The spindle motor control system 24determines the optimum spindle rotational speed according to the radialposition of the optical system and a desired linear velocity to controlthe spindle motor.

In the conventional step of exposing a resist layer of an organicresist, focusing on the resist layer is not performed with a lightsource for exposure itself. An organic resist has chemicalcharacteristics that change continuously by exposure; even weak lightfor focusing is unnecessarily reflected to expose the organic resistlayer. Therefore, focusing is performed with an additional light sourceinsensitive to an organic layer, for example, a red light source with awavelength of 633 nm. The conventional exposure apparatus for organicresists, including two light sources of different wavelengths, requiresan optical system for separating light beams of different wavelengths,and thus has drawbacks such as extreme complexity and higher costs.Moreover, in the conventional exposure apparatus for organic resists,the resolution of the focus error signals for vertical position controlof the objective lens is proportional to the wavelength of the lightsource for detection (for example, a wavelength of 633 nm). Theconventional exposure apparatus, therefore, cannot provide a resolutionas yielded by the light source for exposure, thus precludinghigh-precision and stable focusing.

On the other hand, an inorganic resist material of the present inventionexhibits a steep change in chemical characteristic by exposure, as shownin FIG. 2, which indicates the relation between the irradiation power ofthe light source for exposure and the difference in etching rate betweenexposed and unexposed areas (contrast). In other words, an exposuresource does not unnecessarily expose the inorganic resist in irradiationpower below an irradiation threshold power P0 at which the exposurestarts, even if the irradiation is repeated. This permits focusing withan exposure source of irradiation power below P0 . The nanofabricationmethod of the present invention, therefore, eliminates use of theoptical system for separating laser beams of different wavelengths tooffer a low-cost exposure apparatus and high-precision focusingcorresponding to the exposure wavelength, thus leading to precisenanofabrication. In addition, the inorganic resist material of thepresent invention, which is not exposed by weak light below theirradiation threshold power P0 , does not need to block ultravioletlight from room lighting, unlike normal processes for organic resists.

After focusing light below the irradiation threshold power P0 asdescribed above, the turntable 11 is moved to a desired radial position.In this case, the turntable 11 is moved to change the exposure positionon the resist substrate 1 while the position of the optical systemincluding the objective lens 15 is fixed across the plane.Alternatively, the position of the optical system may be changed whilethe turntable 11 holding the resist substrate 1 is fixed.

The beam source 12 emits laser light while the turntable 11 is rotatedto expose the resist layer. This exposure forms a latent image of fineirregularities, which is a spiral groove in recording discs, by movingthe rotating turntable 11 continuously at a low rate across the radiusof the resist substrate 1. In optical discs, meandering irregular pitsand grooves for information data are formed as the latent image of fineirregularities. For production of discs having concentric tracks, suchas magnetic hard discs, the turntable 11 or the optical system is movedstepwise not continuously.

Under the above settings, the beam source 12 emits pulsed or continuouslight having a desired power not less than the irradiation thresholdpower P0 and corresponding to pits or grooves according to informationdata to expose the resist layer sequentially from a desired position ofthe resist substrate 1. Examples of the pulsed light are shown in FIGS.3A and 3B; an example of the continuous light is shown in FIG. 3C.

The resist materials of incompletely oxidized transition metalsaccording to the present invention undergo a change in chemicalcharacteristic by irradiation of ultraviolet or visible light having theirradiation threshold power P0 or more to produce selectivity, that is,a difference in etching rate to alkali or acid between exposed andunexposed areas.

As the irradiation power decreases, a shorter and narrower pit can beformed, although extremely low irradiation power closing to theirradiation threshold power impairs stable pattern formation. Therefore,the exposure requires the optimum irradiation power in each case.

The present inventors actually confirmed that the selectivity wasprovided by a combination of the resist material of the presentinvention and exposure using a red semiconductor laser with a wavelengthof 660 nm and a mercury lamp with peaks at wavelengths of about 185 nm,254 nm, and 405 nm to form a fine pit pattern.

[Step of Developing Resist Layer]

After the pattern exposure, the resist substrate 1 is developed to forma resist master for optical discs. This resist master has fineirregularities of pits and grooves corresponding to the predeterminedexposure pattern.

In development, a wet process using a liquid such as acids or alkalisprovides the selectivity. The liquid can be properly selected dependingon, for example, the purpose, application, and apparatus. Examples ofalkaline developers for the wet process include a tetramethylammoniumhydroxide solution and inorganic alkaline aqueous solutions containing,for example, KOH, NaOH, and Na₂CO₃; examples of acid developers for thewet process include hydrochloric acid, nitric acid, sulfuric acid, andphosphoric acid. In addition to the wet process, the present inventorsconfirmed that development can be achieved through a dry process such asplasma etching or reactive ion etching (RIE) by controlling the kindsand mixing ratio of gases.

A method for controlling the exposure sensitivity will now be described.For example, in terms of composition W_(1-x)O_(x), a transition metaloxide represented by chemical formula WO₃ exhibits high exposuresensitivity for a range of 0.1<x<0.75, wherein x=0.1 indicates acritical value which causes disadvantages such as a larger irradiationpower necessary for the exposure step and a longer time for development.The highest exposure sensitivity is achieved by x in the range of about0.4 to 0.7.

In terms of composition Mo_(1-x)O_(x), a transition metal oxiderepresented by chemical formula MoO₃ exhibits high exposure sensitivityfor a range of 0.1<x<0.75, wherein x=0.1 indicates a critical valuewhich causes disadvantages such as a larger irradiation power necessaryfor the exposure step and a longer time for development. The highestexposure sensitivity is achieved by x in the range of about 0.4 to 0.7.

In terms of composition Mo_(1-x)O_(x), a transition metal oxiderepresented by chemical formula MoO exhibits high exposure sensitivityfor a range of 0.1<x<0.5, wherein x=0.1 indicates a critical value whichcauses disadvantages such as a larger irradiation power necessary forthe exposure step and a longer time for development.

Higher exposure sensitivity of a resist material has advantages such asa lower irradiation power for the exposure and a shorter exposure timecorresponding to the pulse width or linear velocity. However,excessively high sensitivity has disadvantages such as unnecessaryexposure during the focusing and adverse effects from the lightingenvironment in the process room. Therefore, the optimum exposuresensitivity is selected depending on the purpose in each case. Not onlychanging the oxygen content but also adding another transition metal toan incompletely oxidized transition metal is effective in controllingthe exposure sensitivity of the resist material according to the presentinvention. For example, addition of Mo to W_(1-x)O_(x) improves theexposure sensitivity by about 30%.

The exposure sensitivity can be controlled by not only changing thecomposition of a resist material but also selecting the material for thesubstrate or performing pre-exposure treatment on the substrate. Infact, a study was performed on the difference in exposure sensitivitybetween different kinds of substrates using quartz, silicon, glass, andplastic (polycarbonate) as the substrate. The study confirmed that theexposure sensitivity depends on the kind of substrate and, specifically,the order of the sensitivity is silicon, quartz, glass, and plastic.This order corresponds to that of thermal conductivity; a substrate withlower thermal conductivity exhibits higher exposure sensitivity. This isbecause a substrate with lower thermal conductivity causes a largerincrease in temperature during the exposure and thus a large change inthe chemical characteristics of the resist material.

Examples of the pre-exposure treatment include forming an intermediatelayer between a substrate and a resist layer, heat treatment, andultraviolet light irradiation treatment.

In particular, for substrates with high thermal conductivity, such as asilicon wafer of single crystal silicon, the exposure sensitivity can beproperly improved by forming an intermediate layer with relatively lowthermal conductivity on the substrate. This is because the intermediatelayer improves the thermal charge of the resist material during theexposure. Materials suitable for the intermediate layer with low thermalconductivity include amorphous silicon, silicon dioxide (SiO₂), siliconnitride (SiN), and alumina (Al₂O₃). The intermediate layer may be formedby sputtering or other evaporation processes.

Another substrate is made of quartz, is coated with an ultravioletcurable resin having a thickness of 5 μm by spin coating, and isirradiated with ultraviolet light to cure the liquid resin. Thesubstrate was observed to have higher exposure sensitivity than anuntreated quartz substrate. This can also be explained by the fact thatthe ultraviolet curable resin has thermal conductivity as low as that ofa plastic material.

Furthermore, the exposure sensitivity may be improved by pre-exposuretreatment such as heat treatment and ultraviolet light irradiation. Thisis because the pre-exposure treatment, incompletely and to some extent,change the chemical characteristics of the resist material of thepresent invention.

As described above, incompletely oxidized transition metal resists havea variety of characteristics determined by, for example, materialcompositions, development conditions, and the selection of a substrate.Furthermore, a two-layer resist process is outstandingly effective forextending a range of applications as a resist material. An outline ofthe two-layer resist process will now be described with reference toFIGS. 4A to 4D.

Referring to FIG. 4A, a second resist layer 32 is deposited on asubstrate 31, and then a first resist layer 30 of an incompletelyoxidized transition metal according to the present invention isdeposited on the second resist layer 32. The second resist layer 32 ismade of a material that provides significantly high selectivity to theincompletely oxidized transition metal of the first resist layer 30.

Referring to FIG. 4B, the first resist layer 30 is exposed and developedto be patterned.

Referring to FIG. 4C, the second resist layer 32 is etched through themask pattern of the first resist layer 30 under etching conditions ofhigh selectivity to transfer the pattern of the first resist layer 30 tothe second resist layer 32.

Referring to FIG. 4D, the first resist layer 30 is finally removed tocomplete the patterning of the second resist layer 32.

In applying a two-layer resist process to the present invention, forexample, substantially infinite selectivity will be yielded between afirst resist layer of an incompletely oxidized transition metal and asecond resist layer by, for example, RIE or plasma etching using afluorocarbon gas, quartz as a substrate, and a transition metal, such asCr, as the second resist layer.

The nanofabrication method of the present invention, as described above,uses resist materials of the above incompletely oxidized transitionmetals. This results in the advantage that exposure can be carried outby a combination of an inorganic resist and ultraviolet or visiblelight. These inorganic resists are quite different from conventionalones in that the conventional inorganic resists cannot be combined withultraviolet or visible light as an exposure source because theconventional inorganic resists are optically transparent to ultravioletor visible light, and therefore require an expensive exposure apparatussuch as an electron beam or ion beam apparatus.

Moreover, the nanofabrication method uses ultraviolet or visible lighthaving a high drawing speed to greatly reduce the time required forexposure as compared to nanofabrication methods using the conventionalinorganic resists and electron beams.

Furthermore, the nanofabrication method uses the inorganic resistmaterials of incompletely oxidized transition metals to provide a clearpattern at the boundary between exposed and unexposed areas, therebyachieving high-precision nanofabrication. In addition to this, inexposure, the nanofabrication method permits focusing by an exposuresource itself, leading to high resolution.

To form a fine pattern, as described above, the nanofabrication methodof the present invention employs an approach to decrease theproportional constant K in the relation represented by P=K·λ/NA. Thisapproach is different from the conventional approach to achievenanofabrication at a shorter exposure wavelength λ and a largernumerical aperture NA of the objective lens. This approach allowsformation of a finer pattern using exposure apparatuses currently inuse. Specifically, the present invention allows the proportionalconstant K to be below 0.8 and the minimum nanofabrication cycle f of aworkpiece to be reduced as follows:f<0.8λ/NA

The present invention, therefore, achieves the provision of inexpensivedevices allowing direct use of exposure apparatuses currently in use aswell as finer processing.

EXAMPLES

Examples of the present invention will now be described according to theexperimental results.

Example 1

In Example 1, a resist master for optical discs was prepared withincompletely oxidized sexavalent tungsten as a resist material.

A resist layer of incompletely oxidized tungsten was uniformly depositedon a glass substrate having a sufficiently smooth surface by sputtering.The sputtering was carried out with elemental tungsten as a sputteringtarget in an argon-oxygen mixed atmosphere, of which the oxygen gasconcentration was changed to control the degree of oxidation of theincompletely oxidized tungsten.

The deposited resist layer was analyzed with an energy dispersive X-rayspectrometer (EDX) to give x=0.63 in terms of composition W_(1-x)O_(x),wherein the resist layer was 40 nm thick; and the dependence of therefractive index on the wavelength was measured by spectroellipsometry.

The substrate covered with the resist layer was disposed on theturntable in the exposure apparatus shown in FIG. 1. The resist layerwas irradiated with a laser having an irradiation power below theirradiation threshold power while the turntable was rotated at apredetermined rotational speed. The vertical position of the objectivelens was adjusted with an actuator so that the laser was focused on theresist layer.

While the optical system was fixed, the resist layer was moved togetherwith the turntable to a desired radial position by the feed mechanismprovided on the turntable and was irradiated to be exposed with pulsedlight corresponding to pits according to information data. The exposurewas carried out while the rotating turntable was moved continuously andslightly across the radius of the resist substrate, where the exposurewavelength was 0.405 nm; the numerical aperture NA of the exposureoptical system was 0.95; the linear velocity during the exposure was 2.5m/s; and the irradiation power was 6.0 mW.

After the exposure, the resist substrate was developed through a wetprocess with an alkaline developer. In the developing step, the resistsubstrate was developed in the alkaline developer while ultrasonic waveswere applied to ensure the uniformity of etching. The developedsubstrate was sufficiently washed with pure water and isopropyl alcoholand then was dried by, for example, an air blast to finish the process.A tetramethylammonium hydroxide solution was used as the alkalinedeveloper and the developing time was set to 30 minutes.

FIG. 5 shows a developed resist pattern, which was observed with ascanning electron microscope (SEM). In FIG. 5, pits correspond to theexposed areas and are concave with respect to the unexposed area of theresist layer. This shows that the resist material of incompletelyoxidized tungsten functions as a positive resist. Namely, in the resistlayer of incompletely oxidized tungsten, the etching rate at theunexposed area was lower than that at the exposed areas so that theunexposed area of the resist layer substantially kept the depositedthickness after the development. In contrast, the exposed areas of theresist layer were removed by etching to reveal the surface of the glasssubstrate at these areas.

The minimum pit size of the four pits shown in FIG. 5 was 0.15 μm wideand 0.18 μm long. This shows that the nanofabrication method using theresist material according to the present invention significantlyincreases resolution compared with the conventional organic resists,which probably have a pit width of 0.39 μm. In addition, FIG. 5 showsthat the edges of the pits are very clear.

Furthermore, the width and length of the pits after the developmentvaried with the irradiation power and pulse width of the exposuresource.

Example 2

In Example 2, a resist master for optical discs was prepared withincompletely oxidized sexavalent molybdenum as a resist material.

Example 2 employed nearly the same process as Example 1 except thatmolybdenum was used as a sputtering target. Through the process, aresist layer of incompletely oxidized molybdenum was deposited on aglass substrate, was exposed, and was developed to form pits, as shownin FIG. 6. The deposited resist layer was analyzed with an EDX to givex=0.59 in terms of composition Mo_(1-x)O_(x).

In contrast with incompletely oxidized tungsten, the resist layer of theincompletely oxidized molybdenum forms pits at exposed areas, which areconvex with respect to an unexposed area, as shown in FIG. 6. This isbecause the incompletely oxidized molybdenum functions as a negativeresist to the tetramethylammonium hydroxide solution.

Comparative Example 1

In Comparative Example 1, a resist master for optical discs was preparedwith completely oxidized trivalent tungsten as a resist material.

A resist layer of completely oxidized tungsten was deposited on a glasssubstrate by sputtering. The deposited resist layer was analyzed with anEDX to give x=0.75 in terms of composition W_(1-x)O_(x). In thisconnection, electron diffraction analysis using a transmission electronmicroscope revealed that the crystalline state of incompletely oxidizedmonovalent tungsten was amorphous before exposure.

This resist layer was exposed at irradiation power equivalent to that inExamples 1 and 2 or sufficiently high irradiation power. However, theresist layer did not provide selectivity more than 1, thus failing toform a desired pit pattern. That is, the completely oxidized tungsten,optically transparent to the exposure source, had low absorption, whichprecluded the chemical changes of the resist material.

Comparative Example 2

In Comparative Example 2, a resist master for optical discs was preparedwith completely oxidized trivalent molybdenum as a resist material.

A resist layer of completely oxidized molybdenum was deposited on aglass substrate by sputtering. The deposited resist layer was analyzedwith an EDX to give x=0.75 in terms of composition W_(1-x)O_(x).

This resist layer was exposed at irradiation power equivalent to that inExamples 1 and 2 or sufficiently high irradiation power. However, theresist layer did not provide selectivity more than 20.1, failing to forma desired pit pattern. That is, the completely oxidized molybdenum,optically transparent to the exposure source as well as the completelyoxidized tungsten, had low absorption, which precluded the chemicalchanges of the resist material.

Example 3

In Example 3, a resist master for optical discs was prepared withincompletely oxidized sexavalent tungsten and molybdenum as resistmaterials, and an optical disc was finally prepared. FIG. 7 shows anoutline of the preparation process.

An amorphous silicon intermediate layer 101 having a thickness of 80 nmwas uniformly deposited on a substrate 100 of a silicon wafer bysputtering. A resist layer 102 of incompletely oxidized tungsten andmolybdenum was further uniformly deposited on the substrate 100 bysputtering (FIG. 7( a)). The sputtering was performed in an argonatmosphere with a sputtering target of incompletely oxidized tungstenand molybdenum. The deposited resist layer was analyzed with an EDX togive 80:20 in the ratio of tungsten to molybdenum contained in thedeposited incompletely oxidized tungsten and molybdenum, and 60 atomicpercent in oxygen content. The resist layer had a thickness of 55 nm.Electron diffraction analysis by a transmission electron microscoperevealed that the crystalline state of the incompletely oxidizedmonovalent tungsten and molybdenum were amorphous before exposure.

In subsequent steps including exposure of the resist layer, a resistmaster 103 for optical discs was prepared as in Example 1 except forexposure conditions (FIG. 7( b) and (c)). The exposure conditions inExample 3 are as follows: exposure wavelength: 0.405 nm; numericalaperture NA of exposure optical system: 0.95; modulation: 17 PP; pitlength: 112 nm; track pitch: 320 nm; linear velocity during exposure:4.92 m/s; exposure irradiation power: 6.0 mW; writing: simplifiedwriting, the same as that for phase change discs.

FIG. 8 shows an example of the resist master for optical discs afterdevelopment, which was observed by SEM. The resist material ofincompletely oxidized tungsten and molybdenum functioned as a positiveresist. FIG. 8 shows that pits correspond to exposed areas and areconcave with respect to the unexposed area of the resist layer. Thelength (diameter) of the pits formed was about 130 nm. This pit lengthis below the minimum pit length of 170 nm (0.17 μm), which is requiredfor a high-density optical disc with a recording capacity of 25 GB perside. The resist pattern had pits of the same shape at constant pitchesof 300 nm in the pit line direction and 320 nm in the track direction,showing that pits can be formed stably.

A metal nickel film was then deposited on the surface having theirregular pattern of the resist-pattered master by electroplating (FIG.7( d)). The resist master was separated from the film, which wassubjected to a predetermined process to provide a stamper 104 formolding (FIG. 7( e)). The stamper 104 had the same irregular pattern asthat of the resist master.

The stamper for molding was used to duplicate a resin disc 105 ofpolycarbonate, which is a thermoplastic resin, by injection molding(FIG. 7( f)). The irregular surface of the resin disc was then coveredwith a reflective film 106 of AL alloy (FIG. 7( h)) and a protectivefilm 107 with a thickness of 0.1 mm to form an optical disc 12 cm indiameter (FIG. 7( i)). The above steps of producing the optical discusing the resist master were the known art.

FIG. 9 shows an example of a pit pattern observed by SEM on a surface ofthe above optical disc. The pit pattern included pits such as ones witha length of 150 nm and linear ones with a width of 130 nm, whichcorresponded to the actual signal pattern. This indicates that theoptical disc had a recording capacity of 25 GB.

An eye pattern of RF signals was then read out from the above opticaldisc under the following conditions:

-   -   tracking servo: push-pull method    -   modulation: 17 PP    -   pit length: 112 nm    -   track pitch: 320 nm    -   readout linear velocity: 4.92 m/s    -   readout irradiation power: 0.4 mW        FIG. 10 shows the signal evaluation of the eye pattern.

The eye pattern as read out (FIG. 10A) was processed by conventionalequalization to provide an eye pattern (FIG. 10B) showing a jitter valueof 8.0%, and processed by limit equalization to provide an eye pattern(FIG. 10C) showing a jitter value of 4.6%. These sufficiently low valuesare satisfactory results in practice for a ROM disc having a recordingcapacity of 25 GB.

INDUSTRIAL APPLICABILITY

As is obvious from the above description, the resist material accordingto the present invention is made of an incompletely oxidized transitionmetal that absorbs ultraviolet or visible light. The resist material,therefore, allows exposure with exposure apparatuses currently in use,which use ultraviolet or visible light as an exposure source.Furthermore, the incompletely oxidized transition metal, having a smallmolecular size, is used as a resist material to provide an excellentedge pattern, permitting high-precision patterning.

Accordingly, the nanofabrication method using such a resist material cansimultaneously achieve the provision of inexpensive devices and finerprocessing.

1. A resist material comprising an incompletely oxidized transitionmetal having an oxygen content lower than the stoichimetric oxygencontent of the completely oxidized transition metal, wherein theincompletely oxidized transition metal is sexavalent and satisfies theequation 0.1<x<0.75 in terms of composition A_(1-X)O_(X), wherein Arepresents the transition metal.
 2. The resist material according toclaim 1, wherein the transition metal comprises Mo or W.